1. Field of the Invention
The present invention relates to a stencil mask for electron beam projection lithography and, in particular, to an improved stencil mask for electron beam projection lithography which can prevent the generation of pattern defects due to proximity effects and a method for fabricating such masks.
2. Description of the Background Art
Optical lithography is somewhat limited in its ability to produce a pattern of a size suitable for a highly integrated device. In order to overcome the limited resolution of optical lithography systems, it has been suggested that a non-optical lithography employing an electron beam, ion beam or X-ray as a light source should be employed. In particular there has been a lot of interest in electron beam projection lithography (EPL).
However, non-optical lithography cannot utilize a general exposure mask, such as an exposure mask where a CR pattern is formed on a quartz substrate. That is, the wavelengths of the electron beam, ion beam and X-ray exposure sources are only a few angstroms to a few hundred angstroms, and thus these light sources cannot penetrate the exposure mask.
Therefore, in non-optical lithography processes, especially in EPL, a stencil mask is used as a lithography mask. Here, the stencil mask is classified into an on/off type mask, a membrane mask which does not have an aperture region, or an on/off type mask using a scattering contrast.
The on/off type mask includes an aperture region which the electron beam permeates, and an interception region where penetration of the electron beam is intercepted. The interception region is formed by an absorber layer. Here, a thickness of the absorber layer must be greater than an electron penetration depth so as to absorb electrons in a consistent and dependable manner. Accordingly, a process for forming the absorber layer is complicated.
The membrane mask includes a scattering layer where electrons are scattered at a large angle, and a membrane which the electrons penetrate at a small angle. The membrane mask has been popularly employed in scattering with the angular limitation projection electron lithography (SCALPEL). In SCALPEL, the whole chip is exposed using one mask to improve productivity.
The on/off type mask using the scattering contrast includes an aperture region which electrons penetrate and a scattering layer where the electrons are scattered. The on/off mask using the scattering contrast has been employed for a projection reduction exposure with variable axis immersion lenses (PREVAIL).
The non-optical lithography using the aforementioned masks has a disadvantage in that whole regions of the chip are exposed one mask, thus generating a proximity effect between the adjacent regions that have different pattern densities. As a result, the desired patterns cannot be obtained uniformly in all regions of the chip.
For example, in a DRAM, the pattern density of the cell block, namely cell center region, is typically higher than the pattern density of the cell edge region. Accordingly, when a negative resist is utilized in a non-optical lithography process, the energy intensity of the cell edge region is lower than that of the cell center region, as illustrated in FIG. 1. Thus, critical dimensions in the actual pattern tend to be smaller in the cell edge region, thereby tending to cause a pattern collapse due to the altered aspect ratios in the cell edge region. Reference numeral 3 denotes a mask pattern, namely a scattering layer.
On the other hand, as depicted in FIG. 2, the different energy intensities of the regions can be corrected according to a method for adjusting a critical dimension of the scattering layer 3 in the mask, in particular the scattering layer 3 disposed in the cell edge region. However, if the material to be exposed is changed or a dose is varied, the reflection electron amount at the cell center region is different from that at the cell edge region. Thus, it is difficult to correct all variations of the critical dimension of the pattern in the cell edge region corresponding to the change of the material or the dose variation.